Most liveable worlds

I just read about a delightful Astrobiology paper called “A Two-Tiered Approach to Assessing the Habitability of Exoplanets.” The paper itself is behind a paywall, so I’m reliant on secondary reporting, but here’s the abstract:

In the next few years, the number of catalogued exoplanets will be counted in the thousands. This will vastly expand the number of potentially habitable worlds and lead to a systematic assessment of their astrobiological potential. Here, we suggest a two-tiered classification scheme of exoplanet habitability. The first tier consists of an Earth Similarity Index (ESI), which allows worlds to be screened with regard to their similarity to Earth, the only known inhabited planet at this time. The ESI is based on data available or potentially available for most exoplanets such as mass, radius, and temperature. For the second tier of the classification scheme we propose a Planetary Habitability Index (PHI) based on the presence of a stable substrate, available energy, appropriate chemistry, and the potential for holding a liquid solvent. The PHI has been designed to minimize the biased search for life as we know it and to take into account life that might exist under more exotic conditions. As such, the PHI requires more detailed knowledge than is available for any exoplanet at this time. However, future missions such as the Terrestrial Planet Finder will collect this information and advance the PHI. Both indices are formulated in a way that enables their values to be updated as technology and our knowledge about habitable planets, moons, and life advances. Applying the proposed metrics to bodies within our Solar System for comparison reveals two planets in the Gliese 581 system, GJ 581 c and d, with an ESI comparable to that of Mars and a PHI between that of Europa and Enceladus.

Walking on Mars?

It reminds me of two things. The first, a little superficially, is the “most liveable cities” lists that get published from time to time. The second is Tim Chalk, who was probably the best teacher I ever had, and turns out to have had astonishing foresight.

You see, in 1987 (± a year) he had our whole class work on a project that sounds almost exactly like this, except that it was done with fictional planets, possibly because humanity didn’t know enough about enough real exoplanets at the time. It was ostensibly an exercise in teaching 10-year-olds about databases, but the database was of descriptions of planets, and the assignment was to decide which one we should try to colonise. Given how long ago this was and how young we were, I’m fuzzy on the details, but I recall having Earth’s vital statistics for reference, comparing the fictional planets to these, and while not quite compiling a Planet Habitability Index, we did rank them in order of likely habitability.

Seeing people do this for real, a quarter century later, is unspeakably cool.

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